1. Field of the Invention
This invention relates to inorganic fibers for use in reinforcing a composite material which are composed of an internal layer and a surface layer and can be colored in various colors, and a composite material which is reinforced with these inorganic fibers and assumes a beautiful color.
2. Description of the Prior Art
Carbon fibers and silicon carbide fibers have previously been used as reinforcing fibers for various composite materials (see Japanese Laid-Open Patent Publication No. 14687/1977). These fibers have poor wetting property with respect to the matrix and the bond strength between the matrix and the fibers is not sufficient. Hence, composite materials reinforced with these fibers do not have sufficient mechanical strength.
Furthermore, since these fibers are black, the composite materials reinforced with these fibers also assumes a black to grayish black color. It is impossible therefore to obtain a composite material of a beautiful appearance by using these fibers unless the surface of the composite material is coated or another resin is laminated to the composite material.
Metals or plastics reinforced with fibers find extensive use in a wide range of products requiring strength and light weight ranging from tennis rackets, fishing tackles, ski stocks, ski edges, racing cars and pipes to aircraft and automobiles. These applications require not only mechanical strength but also fashionability. Reinforcing fibers known heretofore cannot meet these two requirements at the same time.
Ceramics reinforced with fibers are used as various machine parts and structural materials. In these fields too, composites having a beautiful appearance are required. Known fiber-reinforced ceramics do no meet this requirement.
In composite materials obtained by using the known inorganic fibers, the reinforcing fibers tend to be localized during manufacture, and the fibers are distributed sparsely at some parts and densely at other parts. It is difficult therefore to control the volume ratio of the fibers in the composites. In particular, when the volume ratio of the fibers is low, the reinforcing fibers are not uniformly dispersed in the composite materials and the resulting composite materials do not have high mechanical properties.
A composite material reinforced only with inorganic fibers has high strength anisotropy and its strength is high in the lengthwise direction of the fibers, but low in a direction at right angles to it. A composite material reinforced with only short fibers is isotropic but generally has low strength.
It was proposed to use a combination of continuous long fibers and short fibers or whiskers as reinforcing fibers for composite materials. For example, the long fibers are used to reinforce the inside portion of the composite material and the short fibers, to reinforce its outside portion. To produce such a composite, the manufacturing process is complex, and the resulting composite material does not have sufficient strength. There is also a method in which long fibers and short fibers are mixed during prepreg production. According to this method, the short fibers can be applied to the surface of a bundle of long fibers by using a bristle or the like. It is difficult however to apply short fibers to the surface of every individual long fiber, and the quality of the fiber mass becomes nonuniform.